Hydrogen is the simplest known element. A hydrogen atom has only one proton and one electron. It is the most abundant gas in the Universe and accounts for over thirty per cent of the mass of the Sun. The Sun is basically composed of hydrogen and helium. In nuclear fusion, four nuclei of the hydrogen combine to form one nucleus of helium, releasing a large amount of energy.
Under normal conditions, pure hydrogen is a gas made up of diatomic molecules (H2). This gas is not found in large amounts in free form on Earth, however. Despite being one of the most abundant elements on the Earth's surface, it is usually found combined with other elements. Combined with oxygen it produces water (H2O), with carbon yields hydrocarbons such as methane (CH4) or mixtures such as petroleum.
Hydrogen is one of the most promising alternative fuels, because of its high efficiency and low pollution. It can be used for locomotion, heat-generation and in electricity generators (fuel cells) in places not covered by the electricity grid.
Hydrogen can be obtained by various methods. Most of the processes used nowadays to produce hydrogen are based on the use of fossil fuels, while only seven per cent comes from renewable energy sources. Industrial concerns produce the hydrogen they require by a process called “steam reforming”. The high temperatures of the steam split the hydrogen-carbon bonds in natural gas (CH4). This is the most effective way to produce hydrogen, but does require fossil fuels as energy and heat source. This process is therefore not currently profitable for producing hydrogen and at the same time using it as a fuel. Nor is it environmentally sustainable, since the resulting emissions of CO2 into the atmosphere increase the greenhouse effect and the climate change associated with it.
Another way to make hydrogen is by means of electrolysis of water. This consists in separating the water into its basic elements, hydrogen and oxygen, by passing an electric current (2H2O+electricity ->2H2+O2) through it. The standard electrolytic-cell thermodynamic potential for producing hydrogen by electrolysis of water is 1.24 V. However, the operating potential applied in industry for such electrolysis is around 2 V.
The hydrogen produced by electrolysis is very pure. Hydrogen obtained by electrolysis of water is nevertheless at present more expensive than that obtained from natural gas. There have obviously been attempts to improve electrolytic technology in order to reduce the working voltage and, therefore, the production costs. A research line tried was that of using light to provide some of the energy needed for the electrolysis. The resulting process is known as photoelectrolysis. One recent example of application is disclosed in U.S. Pat. No. 6,063,258 (16 May 2000). This patent describes irradiation with a high-pressure mercury lamp of an electrolytic solution containing ferric ions and a semiconductor photocatalyst in suspension based on solid particles of WO3, in order to reduce the ferric ions to ferrous ions. In a subsequent stage, the resulting solution is electrolysed by conventional means to produce hydrogen. Although this allows the voltage applied in the final phase to be decreased, the problem with this procedure is that the currents obtained are very low (less than 1 mA) and the corresponding hydrogen yields measured are only micromoles per hour. The reason for this is that very low concentrations of iron in solution (<1 mM) were used, since increasing the concentration of ferrous ions led to them being reoxidised to ferric ions on the surface of the WO3, thereby wasting most of the incident light energy. Furthermore, WO3 is not an efficient catalyst to capture solar light, which is the preferred source in such applications because it is free, accessible and inexhaustible. That was why a lamp with a high proportion of UV light was used. Both factors nevertheless make this process unviable on an industrial scale.
Another patent that forms part of the state of the art is U.S. Pat. No. 6,368,492 (9 Apr. 2002). This discloses an appliance for generating hydrogen by electrolysis of an aqueous solution of an organic fuel, such as methanol. The electrolyte is a solid-state polymer membrane such as those normally used in fuel cells. Anodic oxidation of methanol calls for lower potential than that of water, thus saving much of the electrical energy needed. The device nevertheless produces CO2 as a residue, and one wonders why the methanol is not simply used directly as a fuel in a conventional engine or in a fuel cell instead of converting it into hydrogen which, in the vast majority of energy applications, will in turn end up being burned.
There are other procedures for producing hydrogen, but either they do not permit high-purity hydrogen to be obtained at low cost or they are based on producing hydrogen from fossil fuels. No method of general application yet exists, therefore, for producing high-purity hydrogen that is industrially profitable but not based on the use of fossil fuels.